The present invention pertains to the field of handheld, induction balanced, metal detectors having ground exclusion and discrimination capabilities.
Induction balanced metal detectors are commonly used for treasure hunting. Two problems have plagued users: false target signals caused by passing the search head loop over mineralized ground, and target signals caused by undesirable targets. Both problems have been dealt with in earlier metal detectors, with varying degrees of success.
Early ground balance metal detectors were difficult to operate, requiring an iterative procedure of adjusting a knob, pulling a trigger operated switch while holding the loop in the air, and lowering the loop to the ground. If the volume of the metal detector's sound signal changed when lowering the loop, the metal detector was not ground balanced. Thus the user repeated the procedure until the detector was ground balanced. Furthermore, when the metal detector was used over ground having a different mineralization, the procedure had to be repeated.
Early induction-balanced metal detectors detected variations in the magnetic field surrounding the search head, responding to all metals, regardless of their composition. Later metal detectors allowed the user to discriminate between ferrous and nonferrous metals. Further advances brought variable discrimination, providing a target signal when the detected object produced a phase that exceeded a user-set value. Variable discrimination allgwed the user to ignore iron objects, such as nails, while still detecting coins. These metal detectors had a single variable discrimination point, accepting, i.e., providing a detection signal to the user in response to, objects causing a phase angle on one side of the selected discrimination point, while rejecting objects causing a phase angle on the otherside. Unfortunately, nickels cause a signal having a smaller phase angle than aluminum pull tabs with respect to a search coil driver signal. If the user set a prior art metal detector to discriminate against pull tabs, the metal detector would not detect nickels. Likewise, if the user set the metal detector to detect nickels, it would also detect pull tabs. Users were thus forced to learn the different "chirps" and "chatters" the metal detector would produce in response to pull tabs, or dig up countless pull tabs to avoid missing nickels.
Handheld metal detectors of the transmit/receive type generally have an audible "pinpoint" feature. When the user has located an apparent target, the metal detector search head is repeatedly passed over the area in decreasing segments, decreasing the "circle of confusion" which contains the target. The metal detector may produce an audible target signal while some inches from the target. As the search head is brought closer to the target, the receive signal increases, increasing the volume of the audible signal.
The receive signal's range of magnitude generally is much greater than the audible signal's dynamic range. Thus, a point is reached when the receive signal increases with no increase in audio signal volume. In the past, metal detectors have handled this problem by supplying a momentary switch for the user to squeeze when the volume reaches a plateau. The present receive signal level is captured and held by a circuit within the audio section of the metal detector. The detector then provides a minimal audio volume in response to the held receive signal's magnitude, with the audio volume increasing when the receive signal increases further. In this manner, an audible signal is produced for increasingly larger receive signals, pinpointing the location of the target.
Such a target pinpointing method requires a degree of skill to use it effectively. Improper pinpointing results in a larger circle of confusion, requiring more digging to locate the target. This may prevent the user from being able to locate targets in grassy areas without unsightly destruction.
What is needed, then, is a metal detector of adequate sensitivity, with the ability to compensate automatically for mineralization of the ground, to respond to changing mineralization, and, furthermore, having the ability to distinguish clearly between different metallic objects, despite their having similar effects on the magnetic field of the search head.